It was demonstrated that ET signaling is the
It was demonstrated that ET-1 signaling is the primary axon guidance mechanism that accounts for sympathetic innervation of the sinoatrial node and myocardium in the developing mouse heart .
In this article, we will review the findings relating to the basic profiles; i.e. molecular features of ET biosynthesis, receptors, intracellular biochemical processes following the receptor activation, and then describe pathophysiological significance of ET. Of particular importance in the advances in ET research is that many potent ET receptor agonists and antagonists have provided evidences showing a critical involvement of ETs in the etiology and pathology of cardiovascular diseases such as chronic heart failure, cardiac hypertrophy and other cardiac diseases, renal diseases, systemic hypertension, cerebral vasospasm and pulmonary hypertension.
Structure of the mRNA/gene and its regulation The human ET-1 gene contains five exons, and four introns. Each of the five exons encodes a portion of preproET-1. Several characteristic regulatory elements are found within the ET-1 gene: motifs of the consensus binding sequence for the transcription factor nuclear factor-1 , four copies of the hexanucleotide CTGGGA, the acute phase reaction regulatory element, and sequences highly homologous to the octanucleotide consensus AP-1/Jun-binding site [2,13]. The presence of an AP-1/Jun-binding site in the 5′-flanking region could explain the rapid induction of ET-1 mRNA following treatment of endothelial FITC, Fluorescein isothiocyanate with phorbol ester. Regulation of expression of ET-1 mRNA has been studied extensively. Increased mRNA levels have been observed after treatment of the cells with growth factors and cytokines such as thrombin , TGF-β , TNF-α , IL-1  and insulin, or with vasoactive substances [18,19] such as norepinephrine, angiotensin II (AII), vasopressin and bradykinin. Although no SSRE (shear stress responsive element) was detected in the ET-1 gene, mRNA expression and ET-1 production in endothelial cells are regulated by fluid shear stress: high shear stress (25 dynes/cm2) sharply decreases mRNA levels [20,21], whereas low shear stress (5 dynes/cm2) increases ET-1 mRNA expression . Pulsatile stretch also causes enhanced production of ET-1 in endothelial cells . Shear stress appears to regulate ET-1 gene transcription via an upstream cis element by a distinct mechanism not dependent on the protein kinase C (PKC) or cAMP pathways . In contrast, the expression of ET-1 mRNA is inhibited by endothelium-derived relaxing factor (EDRF), prostacyclin, and atrial natriuretic peptide presumably via cGMP-mediated inhibition of phosphatidyl inositide metabolism . Heparin also decreases ET-1 mRNA expression via inhibition of protein kinase C . All of these characteristics of the ET-1 gene bear a resemblance to those of so-called early response genes such as various protooncogenes. Sequence analysis of the ET-1 cDNA revealed the existence of a single copy gene that encodes for a precursor, preproET-1. Human preproET-1 is a 212-amino acid protein that is proteolytically cleaved, as in the case of many bioactive peptides, at paired dibasic amino acids. The studies demonstrated that this protease is a furin-like enzyme and that the cleavage of the precursor by this protease is an essential step before, producing a 38-amino acid residue intermediate peptide, termed \"big ET-1\" . Big ET-1 is subsequently cleaved at Trp21-Val22 by another endopeptidase termed endothelin converting enzyme (ECE), which appears to be specific for ET  (Fig. 1). In vascular endothelial cells, ET-1 is secreted via the constitutive pathway, and the rate-limiting step of its biosynthesis is thought to be at the level of transcription . ET-1 gene expression in vascular endothelial cells has been shown to be stimulated by a number of chemical stimuli as described above. The polymorphism of the ET-1 gene has been reported in humans [28,29]. Tousoulis et al reported that the G5665 T polymorphism on ET-1 gene increased significantly the plasma ET-1 levels in the patients with ischemic heart disease . It was considered that the polymorphism of the ET-1 gene might be associated with some cardiovascular diseases such as hypertension, angina pectoris, acute coronary syndrome .